1. Field of the Invention
The present invention generally relates to a method and an apparatus for enlarging an image, and more particularly to a method and an apparatus for efficiently enlarging an image by using edge signal components while ensuring picture quality in a high frequency area.
2. Description of the Related Art
Nowadays, a terminal capable of reproducing a multimedia image including a Digital Multimedia Broadcasting (DMB) has been generalized. When an image is transmitted according to such a trend, the corresponding image contains a huge amount of data because graphs or texts have been mixed with a natural image. Therefore, it is necessary to perform various image processing operations for complicated images. Among these image processing operations, when a user wants to view an image, which is being displayed through a terminal screen, on a larger screen, it is necessary to perform an image processing operation for digitally enlarging the size of the image. For example, a DMB terminal user may also want to overcome the limitation in the screen size of the terminal and enjoy a more vivid image through a large size of an external display unit. That is, in order to output an image of a Quarter Video Graphics Array (QVGA) (320*240) size, which is being currently provided by a DMB terminal, to a large size of an external display unit, image enlargement is necessary.
For such image enlargement, there exists a method for performing interpolation for the R, G, B signals of an image in consideration of the ratio of size of the original image and the image to be enlarged. This will be described with reference to FIG. 1 illustrating the general construction of a system for performing interpolation. R, G, B signals are converted to YUV signals by an RGB-YUV converter 100. Then, an interpolation unit 110 performs interpolation for an entire image, which is input through the RGB-YUV converter 100, by means of an equal interpolation ratio determined by an interpolation ratio determiner 130. The interpolated image is converted to R, G, B signals again by a YUV-RGB converter 120. Herein, when the interpolation is performed in order to digitally enlarge the size of the image, it is an important key point in image enlargement to maintain the degree of definition of an edge in fields, in which edge information is important, such as graphics and texts.
Generally, in relation to R, G, B signals, since respective signals give the same effect in color expression, it is necessary to perform the same interpolation process for respective signal components in order to minimize the deterioration of picture quality. However, when the same interpolation is performed for an entire image as illustrated in FIG. 1, the deterioration of performance for high frequency signals may prominently occur. Herein, the R, G, B signals represent the three primary colors (red, green and blue) of light which an image display unit, such as a Cathode Ray Tube (CRT) and a Liquid Crystal Display (LCD), separates and outputs color tone signals through an RGB format in generating a color tone or an image. The RGB format requires three color tone separation materials at a minimum in order to express a color tone and requires a large memory capacity, but the separation and transmission effect of a color tone is excellent because an expression scheme used by image display units is employed. Further, in a YUV format, the separation and transmission effect of a color tone is weak as compared to the RGB format, but many more color tones can be expressed by means of small pieces of color tone information. In the YUV format, image signals are divided into luminance signals Y and color tone signals U and V, which represent horizontal and vertical synchronization signals, and are then processed.
In addition to the interpolation process as described above, according to another method for minimizing the deterioration of picture quality, R, G, B signals are converted to YUV signals and the respective signals are processed. Since each of the YUV signals has an independent characteristic, in contrast with the R, G, B signals, they do not have influence on one another. However, when expressing an image, the Y component gives more influence. Accordingly, the Y component is weighted or many more bits are assigned to the Y component, so that an enlarged image of high picture quality can be obtained through reduction of an operation amount or by means of a small memory.
However, when an image has been enlarged, the deterioration of picture quality prominently occurs in a high frequency area, as compared to a low frequency area. For example, since continuity of change in brightness within an image block is maintained but difference in brightness suddenly occurs in a boundary area of blocks, lattice noise may occur. That is, when interpolation for image enlargement is performed for a portion having a great brightness difference between adjacent pixels, the deterioration of picture quality prominently occurs, as compared to a portion having small brightness difference between the adjacent pixels. Further, when enlarging an image, a higher order interpolation method using linear interpolation or spline may be used. When interpolation is performed with no consideration of the frequency components of an image, the deterioration of picture quality may occur when enlarging the image.
In a case in which an image is enlarged by the conventional method as described above, if an image of high picture quality is input, the image is distorted because the original information of the image is not conserved in complicated edge portions having high frequency components. Therefore, the quality of the image may deteriorate.